Sheets and slabs of synthetic mineral appearing material are now commonly used as kitchen countertops and interior and exterior decorative coverings of all kinds for buildings such as banks, air terminals, stores, and the like. Such applications frequently require that the material be fabricated to fit custom designed areas, requiring in turn that the slabs or sheets be butted together or otherwise joined in ways that juxtapose a cross section with a normal surface at 90.degree..
The fabrication process requires extensive time and specially trained craftsmen to be completed successfully, since special tools and procedures are necessary. If a shaped, one piece part of continuous or monolithic material is desired, such a part can only be produced by casting it in a mold cavity under special conditions. In addition to the high costs of such a process and for the installation of the parts (fitting, gluing it in place to a flat sheet, and/or finishing, for example,) there are often color differences between the cast bowl, for example, and the flat slab of the same material.
The sheet (the terms "sheet" and "slab" will be used interchangeably herein) of our invention can provide a relatively complex finished part by a simple thermoforming operation--that is, the sheet is heated and then pulled by vacuum into a concave cavity (or convex) mold, where it is allowed to cool, to retain its new shape. Such a mold can be shaped as a vanity top, with one 90.degree. back splash wall, with a front end bull nose of 1.0 inch radius and a vanity type bowl. After forming, cooling and trimming, the part can be installed directly in place, without additional fabrication required.
Only one contemporary commercial product ("Corian" by DuPont) is said to be capable of being heat bent. However, its performance is not suitable, for example, to make 90.degree. angle back splash wall, since the minimum radius of curvature specified by the "Corian" literature of which we are aware is 3.0 inches.
So far as we are aware, the use of alumina trihydrate in polymethylmethacrylate ("PMMA") articles was first proposed by Stevens et al in U.S. Pat. No. 3,563,939 (col. 4, lines 28-29) and Duggins in Canadian Patent 916,337. Its flame retardant properties are now well known and accepted, and alumina trihydrate ("ATH") is now widely used as a filler in various resinous products. Somewhat more detail for the construction of synthetic mineral products is provided by Duggins in U.S. Pat. No. 3,847,865; crosslinking agents are mentioned, for example. Also proposed are mold release agents, and viscosity reducers such as aliphatic acids.
Buser et al, in U.S. Pat. Nos. 4,085,246 and 4,159,301 address the problem of the settling rates of various particles used in making a simulated granite having a matrix of polymerizable methyl methacrylate ("MMA") having PMMA dissolved in it. See column 7, lines 42-62 of the '301 patent. They use the PMMA to adjust viscosity, which in turn controls the settling rates of the larger particles --see the Examples, particularly Example 5 of U.S. Pat. No. 4,159,301, lines 31-34. They also use chain-transfer agents as accelerators for the polymerization--col. 8, lines 58-68 of the same patent.
Uniformity of color is mentioned as a goal in Gavin et al U.S. Pat. No. 4,413,089, wherein iron oxide pigment of 10 microns or less is uniformly distributed in a syrup of MMA/PMMA which is then cured; prolonged storage of the syrup is not recommended (col. 2, lines 50-64).
In addition to meeting the above-described challenges, a material destined for use as a kitchen countertop, for example, should have a surface which is easily repairable and restored to its original appearance, such as by sanding and polishing, be protected against flammability, and have good temperature resistance in spite of being thermoformable.
The prior art has more or less neglected the goal of thermoformability or thermobending of solid surface sheets, since the prior art products were generally designed for reproducing the look of flat, natural, mineral based sheets.